Accommodation with and without S–cones and chromatic aberration.

Abstract

Abstract: : Purpose: To determine whether short–wavelength–sensitive cones contribute to dynamic accommodation. Methods: Accommodation was monitored continuously while observers (23) viewed a vertical sine–wave grating (2.2 cycles/deg.; 0.53 contrast) in a Badal optical system. The grating target moved sinusoidally toward and away from the eye (0.2 Hz) between 1.25 D and 3.25 D during trials lasting 40.96 sec. There were three illumination conditions: 1. Monochromatic 550 nm light (10 nm bandwidth; 30 cd/m²) to stimulate L– and M–cones without chromatic aberration; 2. Monochromatic 550 nm light (15 cd/m²) + 420 nm light (10 nm bandwidth; 15 cd/m²) to stimulate L–, M– and S–cones with normal longitudinal chromatic aberration (LCA); 3. Monochromatic 550 nm light (15 cd/m²) + 420 nm light (10 nm bandwidth; 15 cd/m²) to stimulate L–, M– and S–cones, viewed through an achromatizing lens to eliminate LCA. The three conditions were presented ten times each in random order. Mean gains and mean accommodation levels for the three conditions were tested by ANOVA. Results: Mean dynamic gains for the three conditions were not statistically different from each other (F=1.66; p=0.197). However mean accommodation level changed significantly (F=7.39; p=0.001). Mean level decreased from 1.8 D to 1.5 D when short–wavelength light was added to the stimulus with normal LCA (p<0.001). When LCA was neutralized the mean level increased to 2.4 D (p<0.001). Conclusions: S–cones did not improve the dynamic accommodation response at 0.2 Hz, most likely because the accommodation response from S–cones is much slower than the response from L– and M–cones. This is in line with our previous finding that latencies and time–constants of accommodation for isolated S–cones are significantly longer than for L– and M–cones. The reduction in mean accommodation level when blue light was added to the stimulus together with LCA could come from a chromatic signal [S– (L+M)] that represents the difference between S– and LM–cone contrasts. With LCA present, blue light was focussed in front of the retina while green focussed behind. Thus the mean level could represent a balance between S– and LM–cone contrasts. The increase in mean accommodation level without LCA can be attributed to the achromatizing lens.

Keywords: photoreceptors: visual performance • refractive error development • refraction